Stripe - check if a customer exists

There is no check on Stripe's end to ensure uniqueness of customers (email, card, name, etc,) and this is something you have to do on your end.

Usually, when you create a customer with a specific email address you need to associate the customer id you got back from the API with the email address. Then next time, you check whether this email address is already in your system and either create a new customer or re-use the customer id from the previous time.

You may check if the customer exists or not by calling GET /customers with the email as a form-urlencoded parameter. You will get 200 OK response but the returned data[] will be empty if this customer email is not there.

https://stripe.com/docs/api/customers/list